This invention relates generally to a thermometer circuit and more particularly to an oscillator circuit which depends upon the difference between two frequencies to detect and indicate temperature. Conventional thermometers use as a temperature sensor a thermistor, a thermocouple, the base-emitter junction of a transistor, a quartz crystal vibrator having a thickness-shear mode of vibration, and others. The temperature sensors, excepting the quartz crystal vibrator, are required to convert a voltage, or an analog quantity that is a function of a measured temperature to a corresponding digital quantity. The measuring device consumes a high level of energy and is large in size. With the quartz crystal vibrator, temperatures can basically be measured with a frequency counter as the quartz crystal vibrator allows a frequency to be detected in proportion to a temperature. The thickness-shear mode vibrator, however, is large-sized, vibrates at a high frequency of several megahertz, and consumes a large amount of electrical power. These are undesirable features for a thermometer.
It is desirable to provide a circuit and a temperature measuring device which generate two resonant frequencies and use the varying difference between the two resonant frequencies as an indicator of temperature. Central to such a circuit is a vibrator such as a quartz crystal vibrator, a ceramic vibrator or the like which should have a Q of 10,000 or higher. The oscillator circuits include tuned elements for the respective oscillation frequencies and circuit means for mixing the frequencies so as to obtain a difference frequency. A difficulty in earlier circuits occurs when the two oscillation frequencies are close to each other, or when the ratio of the difference frequency to the vibration frequencies is in the order of 1/1000. Then it is necessary that the tuned circuits should have a Q of 1,000 or more. Nevertheless, there is difficulty for the tuning circuits, generally LC tuned circuits, to separate the two frequencies. When the difference between the frequencies varies greatly, as is desirable in a sensitive temperature measurement, the range of difference frequencies in comparison with the frequency of the half-width of the tuned circuits necessitates that the coils and capacitors of the tuned circuits be replaced with other inductances and capacitances of different values. The result is a large sized instrument including many costly components.
What is needed is an oscillator circuit for a thermometer which is small in size, low in cost, and extremely accurate over a wide range of operation.